AF 1410 (U.S. Pat. No. 4,076,525) and AirMet 100® (U.S. Pat. No. 5,087,415) are exemplary of alloy steels (Group A herein), sometimes referred to as superalloys, which combine very high strength and high toughness. One drawback of these alloy steels is their very high cost, because their superior mechanical properties are achieved through the use of scarce and costly alloying elements such as cobalt, nickel and molybdenum, and complex processing, namely double vacuum remelting, aging, refrigeration (Table 3). Their high prices and poor machinability limit their applications to mainly military and aerospace products.
U.S. Pat. No. 3,713,905 and U.S. Pat. No. 6,187,261 are exemplary of high strength steels such as grades AISI 4340 and 300M moderately priced alloy steels (Group B herein). They use traditional chemistries to provide high ultimate and yield strengths after thermal processing, but lack the high toughness of the first (Group A) described alloys.
Typical lower priced martensitic alloy steels (Group C herein) as described in U.S. Pat. No. 5,180,450 provide good toughness but lack the high ultimate and yield strengths of the Group A and B alloy steels.
The use of the Group A ultra high strength and high toughness alloy steels is limited because of their prices. Engineers must select from a large numbers of steel alloys to meet either strength or toughness design requirements and cost objectives. This practice causes high steel making and inventory costs and a need for large amounts of scarce high cost alloying elements. Moderately priced steels combining high strength and high toughness (as Group A) with good machinability and heat treatment properties (as Group B) would have substantial economic value and advance the art of alloy steels.